A filter can be used to filter a fluid comprising a liquid and impurities. For example, in a fuel system, a filter is often used to remove water from a liquid fuel to avoid negative effects to moisture-sensitive components (e.g., fuel injection equipment). Additionally or alternatively, the filter removes contaminants which can be damaging to the same or other components (e.g., engine parts). Typically, the filter comprises a filter element having an inlet side to which the fluid flows, and an outlet side from which the liquid flows with the impurities filtered therefrom. For example, in a filter element having a pleated cylindrical filter media, the radially-outer peaks of the pleats can form the inlet side of the filter element and the radially-inner peaks of the pleats can form the outlet side of the filter element.
In many filtering situations, air will often be intermixed with a fluid as it flows towards a filter element. Air will usually pass easily through a “dry” or “just wet” filter media. However, once a filter media becomes saturated with liquid, the surface tension of the liquid in the filter pores prevents air passage until the pressure drop across the filter media reaches a certain “bubble point” pressure. As a result, air tends to accumulate at the inlet side of the filter media, with the liquid passing only through lower portions of the filter media until the bubble point pressure is reached.
Air accumulation on the inlet side of a filter element can be undesirable for many reasons. For example, when fluid passes only through lower portions of a filter media (because air has accumulated at its upper inlet portions), the system can never be completely full and/or the lower portions are subjected to more wear than the upper portions. Additionally or alternatively, when fluid passes only through certain portions of a filter media, the face velocity of the liquid increases in the fluid-passing portions thereby often causing a decrease in filtration efficiency and/or water-separation (and/or contamination removal) effectiveness. Moreover, an abundance of accumulated air on an inlet side of a filter element can cause restart problems in the system to which the filtered liquid (e.g., fuel) is being provided.